Moving webs and other workpieces are often scanned for defects or known patterns using a machine vision device. Machine vision devices typically employ an illumination system to concentrate a bright light into a narrow line along the workpiece. A line scan camera is directed at the illuminated area to receive light from the web. The camera can be oriented to view a direct reflection or transmission of the image as produced by the illumination light. This is termed a so-called "bright field" image. Conversely, the camera can be oriented off-axis relative to the reflected or transmitted is image light, resulting in a so-called "dark field" image.
FIG. 1 schematically illustrates a machine vision system 20 adapted for use with a wide moving web 22. Width is taken in the direction perpendicular to the page and the web moves in the direction of the arrow 23. The web 22 in this embodiment is a transparent or translucent web in which light is transmitted from one face of the web material to an opposing face. The concepts described are also applicable to an opaque web with certain modifications to be described below. The web 22 is typically wide (about four or more feet wherein width is taken in a direction into the page). A single system 20 can view a web section approximately four feet wide. For web widths greater than four feet, a plurality of identical machine vision systems are arranged side-by-side along the web width. A lamp 24, with an associated power supply 26, generates a bright illumination light that is represented as a series of rays 28. The lamp 24 can be any acceptable bright illumination source such as a metal-halide lamp, a high-pressure sodium lamp, a filament lamp, a laser or other coherent light source, a gas discharge lamp, or an LED array or other semiconductor light source. A 750-watt quartz-tungsten-halogen (QTH) lamp is used in one known illumination arrangement. While not shown, a reflector or shield may be provided so that light emanates only from a predetermined region of the lamp 24. A condenser lens assembly 30 is provided adjacent the lamp 24. In this embodiment, a condenser assembly 30 comprises two or more counterfacing plano-convex lenses 32. The lenses have a circular perimeter, forming a focused beam that projects a circular pattern. At the desired focal point 34 of the rays 28 is positioned the head or "entrance face" of a fiber optic cable 38.
With further reference to FIG. 2, the fiber optic cable assembly is shown in further detail. The head 36 of the cable 38 reveals a fiber optic bundle 40 constructed from a large number of small-diameter fiber optic strands with flat, polished ends. The cable 38 typically comprises a resilient outer covering that encases over three-hundred-thousand 0.002-inch diameter optical fibers. Typically, the fibers are packed to substantially fill the cross-sectional area of the cable 38. The diameter D of the head 36 is approximately 1.35 inches, and is approximately equal to the diameter of the spot generated by the rays 28. In other words, the head 36 is located relative to the condenser lens assembly 30 so that the illumination spot approximately covers the head 36. Note that the amplitude of the distribution within this spot may not be uniform.
At the opposing tail or "exit face" of the cable 38 is located an illumination line housing 42. The line housing 42 includes an elongated rectangular opening 44 that reveals the polished tail ends of the cable's individual optical fibers. The line housing opening 44 and a typical implementation has a width W of four feet and a height H of 0.030 inch. In general, ten to twenty fibers can be aligned vertically side-by-side within the confines of the height H.
The long, narrow line of bundled optical fibers within the line housing 42 generates a corresponding narrow elongated line of light rays 46 that are typically refocused by an elongated focusing lens 48. Randomization of the fibers between the head and tail ends of the cable generally tends to improve the uniformity of the resulting illumination line. In fact, it is common to twist and braid the fibers within a cable to achieve greater uniformity in transmitted light distribution. However, the system may rely on a certain rotational orientation of the head end relative to the illumination source (e.g. the light bulb), and rotation of the head end relative to the source at a later time may disturb the system's settings. In addition, the particular distribution of light produced may limit the ability to subsequently move the camera relative to the image line. Thus the randomization can be both advantageous and problematic.
A variety of lens constructions can be used such as a linear convex cylinder lens or an acrylic rod lens approximately one inch in diameter. An off-axis elliptical reflector can also be employed. The term "lens" should be taken broadly to include a focusing reflector. The lens 48, focuses the rays 46 onto the web 22 to generate the desired narrow illumination line across the width of the web 22. Based upon this illumination line, light is transmitted through the web 22 to a camera assembly 50 that comprises a wide-angle focused lens 52 and an electro-optical pick up assembly, such as a commercially available CCD camera unit 54. The lens 52 is focused on the web 22 using the focus ring 55 and a desired lens aperture is provided using the aperture adjustment ring 57. Images are converted by the CCD camera into electronic signals for transmission to an appropriate data acquisition device 56. The device can comprise a microcomputer having pattern-recognition software for analyzing the surface of the viewed on the web. A monitor 58 can also be provided for real-time of viewing of the web surface or for viewing data derived by the software.
One problem encountered with the machine vision system described above is that the quality of the substantially uniformly illuminated image viewed by the wide-angle lens tends to degrade near the edges of the field of view of the lens. This degradation is due largely to the inherent optical characteristics of wide-angle lenses. FIG. 3 illustrates the camera lens response measured by the CCD element 54 for a 35 mm Nikon Nikkor.TM. lens. The specified field of view of this lens is .+-.32.degree. from perpendicular (0.degree., directly beneath the camera) which translates into approximately four feet of web width at a focal length of approximately 3.2 feet from the web face to the camera'optical plane 53. The measured response curve 52 registers 100 percent intensity at a 0.degree. field of view. In other words, maximum intensity is viewed directly beneath the lens. As the viewing angle increases, the viewed intensity drops off sharply. The decrease in intensity occurs as a function of the cosine to the fourth power of the camera's field of view angle. At maximum viewing angles the intensity of the viewed image is about 52 percent of the maximum value. This uneven response can lead to viewing errors since illumination at the edges of the field of view may be insufficient to acquire a proper image of the web. Another associated problem with the prior art illumination arrangement is that the camera operates most effectively when it is aligned near the center of the illumination line. Placing the camera "off-axis" can result in a significantly degraded acquired image.
There are several techniques currently employed to overcome the imaging limitations of wide-angle lenses. One technique entails providing additional illumination near the edges of each camera's field of view. Cameras can be placed closer together so that the full field of view of each camera is not utilized. This and other solutions to the problem, however, substantially increases equipment cost and reduces efficiency. Conversely, some of the light near the center of the field of view can be attenuated using various types of well-known apodizers so that the overall image appears more-even across the viewing range. These attenuation techniques, however, often reduce the image quality since the total amount of light entering the lens is reduced.
It is, therefore, an object of this invention to provide a method and apparatus for providing an illumination line that enhances the response of a wide-angle lens at the edges of its field of view. This illumination line should not require any attenuation of light entering the camera lens and should not require any decrease in the rated field of view of the lens. Furthermore, the choice of the position of the camera along the illumination line should not influence the benefits of the illumination line provided according to this invention.